1. Field of the Invention
The present invention relates generally to cell death, and more particularly, to methods and compositions for regulating cell death through activating the autophagic pathway.
2. Background of the Related Art
Apoptosis is a well-studied pathway of programmed cell death conserved from C. elegans to humans (1). Caspases, a family of cystinyl, aspartate-requiring proteases produce the morphological changes associated with apoptotic death (2, 3). Non-apoptotic forms of cell elimination include those with features of necrosis and autophagy (4-7). Autophagy is a process that liberates free amino acids and nucleotides and enables cells to survive under nutrient deprivation conditions as well as to undergo structural remodeling during differentiation. Necrosis can result when cell metabolism and integrity are compromised by a nonphysiological insult.
During autophagy, isolation membranes are used to sequester cytoplasmic components, such as proteins and organelles including mitochondria, and target these membrane-bound structures for degradation by fusion with lysosomes (8, 29). Genetic studies of autophagy induced by nutrient starvation in the yeast Saccharomyces cerevisiae resulted in the identification of the ATG genes (30-33). ATG genes are involved in the activation of the signaling complex that triggers formation of autophagic structures from isolation membranes that are known as autophagosomes, and this process involves two ubiquitination-like pathways (8). Autophagosomes then dock and fuse with lysosomes where they are degraded.
Autophagy has been observed in various eukaryotic organisms, and the ATG genes appear to be conserved in organisms, that are as different as yeast and humans (10). The ATG genes appear to be a survival response to nutritional starvation involving membrane-bound vacuoles that target organelles and proteins to the lysosome for degradation (8, 9).
Morphological studies of dying cells during embryogenesis resulted in the identification of at least two prominent forms of physiological cell death (34, 35). These dying cells were distinguished based on how they are degraded and removed dying cells. Cells undergoing autophagy digest themselves by formation of autophagosomes that transport degraded cargo to the lysosome. In contrast, during apoptosis, phagocytes eat dying cells and the dead cells are degraded by the lysosome of the phagocyte. Apoptosis has been a subject of intense investigation in recent years and two general mechanisms has been described for inducing this type of cell death (1). The intrinsic apoptosis pathway relies on the mitochondria for regulatory components that are involved in the activation of caspase proteases that cleave death-inducing protein substrates. The extrinsic apoptosis pathway depends on extracellular death ligands such as TNF and Fas, and these ligands are bound by trimeric death receptors that recruit adapter proteins, such as FADD and TRADD, that recruit caspases 8 and 10. The proximity of these caspases within signaling complexes results in their proteolytic activation and cell death.
Several groups have observed necrosis-like cell death that appears to occur in a caspase independent manner (16). Furthermore, non-apoptotic cell death appears to provide a compensatory mechanism for cell killing when apoptotic regulators such as caspases and Apaf1are compromised (36, 37). Thus, it would be beneficial to determine methods, enzymatic pathways and compounds that induce such non-apoptotic mechanisms to compensate when programmed death by apoptosis is compromised.